1. Field of the Invention
The invention relates to processes for using electrophotographic systems to make and assemble a number of color toned images to give a full color reproduction. More particularly the invention relates to the use of such systems to make accurate color proofs for the printing industry.
2. Background of the Art
Full color reproductions by electrophotography have been generally known for many years (e.g., U.S. No. 2,297,691) but no detailed mechanisms were described and the toners disclosed were dry powders. U.S. Pat. Nos. 2,899,335 and 2,907,674 pointed out that dry toners had many limitations with respect to image quality used for superimposed color images. Liquid toners were recommended for the purpose of improved image quality. These toners comprised carrier liquids which were of high resistivity, e.g. 10.sup.9 ohm-cm or more, with colorant particles dipersed in the liquid, and preferably an additive intended to enhance the charge carried by the colorant particles. U.S. No. 3,337,340 disclosed that one toner deposited first may be sufficiently conductive to interfere with a succeeding charging step. It was claimed that the use of resins which are both insulative (resistivity greater than 10.sup.10 ohm-cm) and a of low dielectric constant (less than 3.5) to cover each colorant particle was necessary to provide good images. U.S. No. 3,135,695 disclosed toner particles stably dispersed in an insulating aliphatic liquid, the toner particles comprising a charged colorant core encapsulated by an aromatic soluble resin treated with a small quantitiy of an aryl-alkyl material.
The use of metal soaps as charge contol and stabilizing additives to liquid toners is disclosed in many earlier patents (e.g. U.S. No. 3,900,412; U.S. No. 3,417,019; U.S. No. 3,779,924; U.S. No. 3,788,995). (Concern has also been expressed and corrective measures offered for the inefficient action experienced when charge control additives or other charged additives migrate from the toner particles into the carrier liquid (U.S. No. 3,900,413; U.S. No. 3,954,640; U.S. No. 3,977,983; U.S. No. 4,081,391; U.S. No. 4,264,699). In U.S. No. 3,890,240 it is disclosed that typical liquid toners known in the art have conductivities in the range 1.times.10.sup.-11 to 10.times.10.sup.-11 mho/cm. GB No. 2,023,860 discloses centrifuging the toner particles out of a liquid toner and redispersing them in fresh liquid as a way of reducing conductivity in the liquid itself. After repeating the process several times the conductivity of the liquid toner was reduced by a factor of about 23 and was disclosed as a sensitive developer for low contrast charge images.
In several patents the idea is advanced that the level of free charge within the liquid toner as a function of the mass of toner particles is important to the efficiency of the developing process. In U.S. No. 4,547,449 this measure was used to evaluate the unwanted charge buildup on replenishment of the toner during use, and in U.S. No. 4,606,989 it was used as a measure of deterioration of the toner on aging. In U.S. No. 4,525,446 the aging of the toner was measured by the charge present and it was shown how the charge was generally related to the zeta potential of the individual particles. U.S. No. 4,564,574, discloses chelating charge director salts onto the polymer, used in liquid toners and discloses measured values of zeta potential on toner particles. Values of 33 mV and 26.2 mV with particle diameters of 250 nm and 400 nm are given. The purpose of the salts is to improve stability of the liquid toner. A literature reference, "Research into the Electrokinetic Properties of Electrographic Liquid Developers", V. M. Muller et al, IEE on Industry Applications, vol. 1A-16, pages 771-776 (1980), treats the liquid toner system theoretically but also gives experimental results on certain toners. Using very small toner particles (all less than about 0.1 micron), zeta potentials in the range 15 mV to 99 mV with related conductivity ratios were used. These latter ratios appear to relate the conductivity of the toner immediately after the current is initiated to the conductivity value after prolonged passage of the current. The former values are believed to contain both toner particle and soluble ionic species conductivities; the latter is believed to be the basic conductivity of the carrier liquid after most of the added charged carriers have been deposited by the current flow. Finally in U.S. No. 4,155,862 the charge per unit mass of the toner was related to difficulties experienced in the art in superposing several layers of different colored toners. This latter problem was approached in a different way in U.S. No. 4,275,136 where adhesion of one toner layer to another was enhanced by an aluminum or zinc hydroxide additive on the surface of the toner particles.
Diameters of toner particles in liquid toners vary from a range of 2.5 to 25.0 microns in U.S. No. 3,900,412 to values in the sub-micron range in U.S. No. 4,032,463 U.S. No. 4,081,391, and U.S. No. 4,525,446, and are even smaller in the Muller paper. It is stated in U.S. No. 4,032,463 that the prior art makes it clear that sizes in the range 0.1 to 0.3 microns are not preferred because they give low image densities.
Liquid toners which provide developed images which rapidly self-fix to a smooth surface at room temperature after removal of the carrier liquid are disclosed in U.S. No. 4,480,022 and U.S. No. 4,507,377. These toner images are said to have higher adhesion to the substrate and to be less liable to crack. No disclosure is made of their use in multicolor image assemblies.
The art therefore discloses an awareness of the importance of the physical parameters of the liquid toner-conductivities, zeta potentials of toner particles, charge per particle or per unit mass of particles, and the localization of the charge on the particles. Most of the references above are concerned with the efficiency of liquid toners in the context of monochomatic image development. Only U.S. No. 4,155,862 and U.S. No. 4,275,136 are explicitly concerned with multicolor toned images, and only the first of these relates the quality of the multicolor toned assembly to the charge per gram of the toner particles.